This invention relates to arc discharge lamps and, more particularly, to novel electrode mounting configurations which reduce the required length of the lamp envelope.
Tubular arc discharge lamps, such as conventional fluorescent lamps, project light upon a surface in a relatively uniform manner except for a gradual decrease in illumination near the ends. This end falloff is ordinarily not a problem when the lamp is used for general purpose lighting. In certain applications, however, such as use as the exposure source in a photocopying machine, the light falloff is compensated for in some manner to obtain relatively uniform illumination of a document to be copied. Various ways of providing for this compensation are known to the art: U.S. Pat. Nos. 3,225,241 and 3,717,781 are representative of the so-called aperture fluorescent lamps which disclose ways of changing the properties of the coatings near the ends of the lamp. In the xerographic art, it is more usual to shape the output light profile of the illumination lamp by interposing a so-called butterfly slit in the optical path between the lamp and the document, the slit shape serving to allow increased illumination at the ends of the document. Whether the compensation is within the lamp itself, or to the light output, there is an inherent penalty in the length of the lamp due to the way in which the lamp electrodes have hitherto been mounted. For example, in a standard fluorescent lamp, the electrodes on each side project into the tube approximately a distance of 1.75" (43.75 mm), i.e. each filament is approximately 13/4" away from the lamp ends. For each tube, there is therefore, a length of 31/2" (87.5 mm) which is providing little or no illumination.